Aircraft construction

ABSTRACT

An aircraft structure for the minimization of microwave energy reflection from the aircraft back to a receiver. The provision of structural configurations and materials operate to reduce microwave energy reflection toward its source or another receiver located at a level below the aircraft and laterally or forward thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 295,644 filed on July 17, 1963, now U.S. Pat. No.4,924,228.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to aircraft construction. More particularly thisinvention relates to the minimization of microwave energy reflectionfrom an aircraft back to a receiver.

2. Description of the Related Art

Minimization of radar reflectivity is of varying importance in differentkinds of military missions. The configuration shown herein is of areconnaissance and surveillance vehicle where survival may be largelycontingent on avoidance of detection. The details described willvariously apply to other aircraft and other military vehicles.

SUMMARY OF THE INVENTION

Among the objects of this invention is the provision of novel structuralconfigurations and material means operating to reduce microwave energyreflection toward its source or another receiver located at a levelbelow the aircraft and laterally or forward thereof.

Another object of this invention is the provision of a novel structureproviding aerodynamic shaping, reflecting microwave energy in adirection away from its source, absorbing microwave energy, providingelectrical shaping in different configurations than the aerodynamicshaping, and/or avoiding or shielding reflective surfaces in theaircraft.

Further objects of this invention reside in the providing of meansdirecting microwave energy reflection upwardly from an aircraft; ofproviding the above means in a structure with sufficient strength at areasonable cost and suitable weight; of providing a novel inlet duct toan aircraft power plant minimizing reflection off the face of the powerplant back toward the microwave energy source; and/or of providingoptimum visibility through a canopy from a cockpit of the aircraft whileminimizing microwave energy reflection therefrom.

BRIEF DESCRIPTION OF THE DRAWING

The invention further resides in certain novel features of construction,combinations, and arrangements of parts and further objects andadvantages of the invention will be apparent to those skilled in the artto which it pertains from the following description of the presentpreferred embodiment thereof described with reference to theaccompanying drawings, which form a part of this specification, whereinthe same reference numerals indicate corresponding parts throughout theseveral views, and in which:

FIG. 1 is a perspective view of a reconnaissance and surveillancevehicle forming a specific embodiment of the invention;

FIG. 2 is a bell-shaped cross sectional view of the vehicle taken alongline 2--2 of FIG. 1;

FIG. 3 is an enlarged view of a lower starboard side corner portion ofthe fuselage cross section shown in FIG. 2;

FIG. 4 is a partial cross sectional view of a port wing leading edgetaken along line 4--4 of FIG. 1;

FIG. 5 is a diagrammatic fragmentary cross sectional view of anatmospheric air inlet duct leading to an airbreathing engine;

FIGS. 6 and 7 are respectively front and side views of a cockpit canopyof the vehicle, showing only the outlines thereof;

FIG. 8 is an enlarged partial sectional view of a canopy construction;and

FIG. 9 is a view of a typical cross section of a wall of the engine airinlet duct.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It is to be understood that the invention is not limited to the detailsof construction and the arrangements of parts shown in the drawings andhereinafter described in detail, but is capable of being otherwiseembodied and of being practiced and carried out in various ways. It isto be further understood that the terminology employed herein is for thepurpose of description and there is no intention to herein limit theinvention beyond the requirements of the prior art.

An aircraft, indicated generally by the reference numeral 10 in FIG. 1,is of a reconnaissance-surveillance type. The structure and materials ofthe aircraft 10 are adapted to minimize radar reflection or at leastradar reflection in the direction of a radar transmitter. Those skilledin the art will understand when similar structures and material systemsshould be used on other types of aircraft. The description will beconfined to those matters having pertinency to the question of radardetection and having novelty over what has previously appeared in theart.

Referring to FIGS. 2 and 3, the cross sectional details are typical ofthose substantially throughout the fuselage area. A fuselage side wall12 forms an angle A to the vertical. In an ideal condition, the aircraftstructure would absorb or transmit all microwave energy. As this is nottotally feasible, there will be some amount of reflection, and the sidewall 12 is preferably configured to reflect in an upward direction theenergy originating laterally of and below the aircraft. The side wall 12would have minimum reflection toward the radar source if angle A were90° to the vertical, the fuselage then being a flat plate. This isassuming that the energy source is coplanar with the plate. Obviously acompromise must be effected so the fuselage can perform normal aircraftfunctions. It would be desirable to have an angle A of 45° to thevertical, but a more suitable compromise is for angle A to be a minimumof 10° and preferably not more than 20° . The drawing shows angle A as17°. This angle will direct reflection upwardly in normal aircraftflight. It is understood that the angle A to the vertical will changewhen the aircraft is banked or rolled.

A fuselage bottom wall 14 preferably would be horizontal but iscompromised so the fuselage can accommodate normal aircraft functions bybeing configured as a curved, relatively shallow convexity. A line 16,lying tangent to the bottom wall 14 adjacent to a corner 18, forms anangle B with the horizontal. The angle B is preferably a minimum valuebut not so low as to impair the normal fuselage functions. An angle Cbetween the side wall 12 and the tangent line 16 preferably, in view ofnecessary compromises, has a value with a range of 75° to 100°. Theangle C is shown in FIGS. 2 and 3 of the drawings as being approximately100°. Understandably, it would be desirable to reduce the angle as muchas possible consistent with other considerations.

The radius of the corner 18 should be minimal as the corner forms asurface reflective back toward a microwave energy source that is locatedlaterally of and below the aircraft, such as defined by a radar sourceline 20. This reflective surface is theoretically a line defined by atangent to the corner 18 at the illustrative radar source line 20 fromits transmitter. A one-inch maximum radius is feasible in most aircraftconstructions. The smaller the radius the better. The radius shown inthe drawing is preferably to be taken as half an inch as this was theradius used in the structure described in FIGS. 2 and 3.

FIGS. 2 and 3 illustrate a typical cross section through the aircraftfuselage. The structural shell of the fuselage, carrying most of theloads, is formed with a honeycomb structure 22. The honeycomb 22 may besimilar to the kind of fiberglas honeycomb presently commonly used inthe aircraft art for various purposes. The honeycomb 22 has glassfibers. The skins of the honeycomb 22 are impregnated with resinousmaterial. Fiberglass honeycomb is preferable to paper honeycomb as itsresistance to moisture is superior. Moisture increases reflectivity ofmicrowave energy. In the various places herein where fiberglasreinforced plastic is mentioned, it will be understood by those skilledin the art that Nylon* or other artificial fibers can be substituted.The structure can be made of sufficient load carrying strength toperform its designated functions in a vehicle, such as is hereindescribed. Other structural details such as fiberglass formers 24 andlongerons 26 can also be made of sufficient strength to perform thedesignated functions thereof.

The shell 22 has a blanket 30 of rigid structural foam. The expression"rigid structural foam" defines a foam having significant load carryingproperties, as distinguished from a flexible foam. It is desirable tofill this foam by a suitable method such as is disclosed in thecopending patent application, Ser. No. 263,704 filed Mar. 8, 1963,entitled "Open Cell Rigid Foam, " Marlan R. Pollock and Marlyn F. Harp,inventors.

The foam blanket 30 preferably has a thickness of approximately oneinch. The foam is filled with a microwave energy absorbing ordissipating material whereby the composite structure absorbs, ratherthan reflects, microwave energy by converting the microwave energy toheat. The blanket 30 preferably has a dielectric constant of between 2and 10. Carbon is a preferred filter but it is possible to use othermaterials, such as iron or nichrome. A filler may be introduced into themixture before foaming but a better structure is achieved if it isabsorbed into an open cell foam in a manner such as is described in theabove patent application. The blanket 30 is bonded in place by asuitable bonding agent. Other than the details set forth herein, theselection of foam and filler, and the techniques of formulation andapplication, are within the skill of the aircraft and chemical materialsand processes people, who are familiar with various considerationsinvolved, including providing suitable strength, weight, economy, and soforth.

In some installations, considerations may dictate placing foam of auniform or graded dielectric constant 30, within the interstices of thehoneycomb 22. The filler is in the form of finely divided particles thatdo not provide a continuously conductive surface. The foam wall tends toinsulate any filler particles. Additionally, at least a part of anyresistance to current flow depend on the nature of the filler. Carbon infinely divided form provides good resistance. The filler is a materialwhich will support a current, e.g., the material selected must have theproper combination of properties as a conductor and as an insulator.Carbon black has proven to be a preferred form of carbon.

One measure of the property of the filler is its resistivity measured inmicroohms per centimeter. Carbon has a resistivity in microohms percentimeter of approximately 1000 to 5000. Aluminum has a resistivity of1-10 microohms per centimeter, and nichrome has a resistivity of 150-200microohms per centimeter. Generally the resistivity should be as high aspossible without effectively operating as an insulator. The operablerange in resistivity is given as 100 to 10,000 microohms per centimeter.

The approximately one inch thickness of the blanket 30 is designed tohave substantial effect in absorbing microwave energy of about 2000 to2600 megacycles (mc) and above. If the thickness were reduced to 1/2inch, the principal effectivity would be about 4000 mc and above. Thelimit on the principal effectivity is that the thickness should be about1/4 wave length. Reflectivity is describable as a curve and is given avalue in effectivity in terms of wave lengths by selecting a value onthe curve whereby a thickness selected for 2000 mc also has someeffectivity for lower values. Any selection is in the nature of acompromise. If the selected thickness exceeds 1/4 wave length, theincrease in absorption for increased from thickness is small.

As before state, the composite structure or blanket 30 preferably has adielectric constant of 2 to 10. The value of the dielectric constant inthe present specific example of a blanket 30 having a thickness of oneinch, may be taken as 6 to 8. The loss tangent preferably should bebetween 0.05 and 1.0 and may be taken as about 0.10 in this specificstructure. Generally, the loss tangent should be as high as possible andthe dielectric constant should be as low as possible. Permeativity is amore basic term but it is convenient to speak of a dielectric constant.The dielectric constant is dependent on the amount and type of filler.The dielectric constant is preferably as low as possible so thatmicrowave energy can enter the structure with the least amount ofdiscontinuity. However, the value of the desired dielectric constant iscompromised to a higher value to maximize energy absorption. As morefully described hereinbelow, the structure of the corner 18 is graded indielectric constant throughout a number of layers. If weight were not aconsideration, the side walls would also have layers of foam graded asto dielectric constants. The side wall 12 is disposed at an anglegreater than ninety degrees to a line 20 from the radar source and thisfacilitates entry of microwave energy into the wall, as the effect whichmight be called a "wedge effect" is to minimize discontinuity.

A particular problem exists at the lower corner 18 because, to a degree,the structure tends to reflect microwave energy back to its radartransmitter from a tangent on the radiused surface normal to the line 20directly. The line or point of tangency changes as the aircraft banks.The solution used here is to maximize absorption of microwave energy atthe corner 18. One factor interfering with energy absorption is thediscontinuity between the dielectric constant of the air and of thestructure. As described below, this discontinuity is minimized by usinga graded structure. Another method of reducing the discontinuity ofmicrowave energy absorption is by altering the wedge shape of the corner18 in the manner shown. The corner 18 results in less discontinuity ofmicrowave energy absorption than a surface normal to the line 20.

A built-up corner structure 34 is provided by bonding from layers 40,42, and 44 to the blanket 30. By way of example, the blanket 30 has adielectric constant of 6 to 8. The layers 40, 42, 44 have dielectricconstants of 3 to 4, 2 to 3, 1.5 to 2, respectively. This gradeddielectric structure is quite effective in absorbing microwave energy.The amount of carbon needed to be absorbed to achieve these dielectricconstants may readily be computed by those skilled in the art fromstandard publications in the field of antennas.

Whereas the corner 18 is left bare of a filter coating 48 for the actionof absorbing energy described herein, above, the areas of blanket 30back from the corner 18 are provided with the filer coating 48externally of a skin 50, that may be said to form the electrical shapeof the fuselage for frequencies above 2,000 mc. The filter coating 48 isconveniently applied as a paint with a pigment concentration of aluminumof approximately 18 ounces per gallon where the coating has a thicknessof 0.0006 inch. The coating 48 acts as a filter in the sense that isreflects a low percentage of microwave energy of frequencies below 2,000mc and a higher percentage of energy of frequencies above 2,000 mc. Insome installations, considerations may dictate integrating the filterwith the skin 50 or placing the filter coating 48 inside the skin 50.

The aluminum particles in the paint pigment of the coating 48 can havenormal aluminum paint grind. This filter of aluminum would act like asheet of metal if the particles did not tend to become electricallyisolated. Such isolation results from oxidation of the aluminum as wellas from some separation by the paint vehicle. Aluminum is the preferredpigment for the filter coating 48, partly because of the self-oxidizingproperties. Other fillers such as silver, and graphite would need tohave an insulator coating separating each particle to achieve the sameisolation as occurs from the oxidation of the aluminum.

An increased concentration of pigment in the paint, an increased coatingthickness, and/or multiple coatings lower the frequency at which thefilter 48 reflects. The filter 48 is preferably designed to reflectabout 85 percent of the microwave energy at about 3,000 mc. In use ofthe filter 48, frequencies much below 2,000 mc are to be transmittedand/or absorbed. For example, if frequencies below 2,000 mc were not ofconcern, flat metal sheets could be used in the side wall 12 as far asvalues of reflection alone are concerned. At microwave frequencies belowabout 2,000 mc, for example at 500 mc, the angular disposition of theside wall 12 is ineffective to reflect microwaves upwardly. Accordingly,this lower range of microwave frequencies are best transmitted orabsorbed.

The skin 50 of all fuselage areas is preferably formed of a typicalaircraft fiberglas, plastic-impregnated sheet. The sheet is about 0.03inch in thickness at the corner 18 and is increased to 0.06 inchthickness on other areas of the side walls 12 and the bottom wall 14. Itshould be noted that within the fuselage, as well as in other areas ofthe aircraft, metallic structures are avoided whenever possible. Thebottom wall 14 is built up of or is a continuation of the same materialsas the side walls 12. The bottom wall 14 is comprised of the shell 22,the filled blanket 30, the filter coating 48 and the skin 50.

FIG. 4 shows a typical wing leading edge cross section. A skin 52 isformed of the same type of fiberglas reinforced plastic sheet as is usedto form the skin 50 on the fuselage. The skin 52, in its preferred form,is 0.03 inch in thickness. The skin is structural supported and itsaerodynamic shape is defined by a rigid structural foam 54. The foam 54may be the same type of material as the foam blanket 30 in the fuselage,but the foam 54 is preferably unfilled.

The inner surfaces of the foam body 54 are coated with a pigmentedfilter 56 of the same type, thickness, and concentration as the filter48 of the fuselage. The filter 56 is interiorly applied to an uppersurface 58 and a lower surface 60 on the inside of the foam body 54. Theupper surface 58 tends to direct microwave reflection upward and thelower surface 60 tends to direct microwave reflection downward relativeto a line 62 from a microwave energy source forward of the aircraft. Anelectrical corner is inside of the foam body 54, and is made sharp sothat the type of problem discussed above with the radiused corner 18 ofthe fuselage is avoided.

A main forward load carrying member of the wing leading edge is alaterally extending spar 64, which is built up from resin impregnatedfiberglas. The foam body 54 is bonded to and supported by the spar 64. Acavity 68 between the foam body 54 and spar 64 is filled with aplurality of filled foam layers 70, 72, 74, and 76. These layers havethe same composition and have the same dielectric constants as thegraded dielectric foams 30, 40, 42 and 44 in the fuselage lower corner18.

The principles embodied in the foregoing fuselage and leading windstructures, for minimizing microwave energy reflection from atransmitter, may be variously applied to the other structures and/orempennage of the aircraft 10, or to other aircraft.

FIG. 5 shows the relationship of an air inlet duct 80 to an airbreathingaircraft engine 81. The functional purpose of inlet duct 80 is to ductatmospheric air to the engine 81. The improvement herein comprises theminimization of the reflection of microwave energy from a forward end orface 82 of the engine 81 back to the microwave energy source. It will beunderstood that a conventional straight, substantially horizontal airinlet duct would result in little if any interference with microwavereflection from the engine face 82. The inlet duct 80 shown in FIG. 56has an ogee curved center line. Accordingly, when the inlet duct 80 isviewed from a forward hemisphere, the engine face 82 is entirely oralmost entirely shielded. It may not be possible in some inlet ductsituations to provide more than a slight bend therein but this importantengine face shielding function of the duct 80 should be applied to theextent feasible. Accordingly, when the inlet duct 80 is viewed fromdirectly forward of the aircraft, a minimum of the engine face 82 shouldbe allowed to be exposed. Ten percent of the engine face area is anacceptable minimum.

The plane defined by the ogee curved center line is preferably verticalwhereby the forward end thereof is above the rear end, as shown in FIG.5. In other aircraft the vertical component of the ogee bend may have tobe reduced towards a zero vertical component if the air inlet duct isdisposed laterally of the aircraft fuselage, depending on the need inthe aircraft to have an air inlet duct that is not top mounted. Thefunction of the vertical component of the curved duct 80 is to directmicrowave reflection to one or more of the duct walls upon entering theduct opening.

The line 84 denotes a beam of microwave energy from a radar transmitterand shows the same striking the interior wall of the air inlet duct 80 anumber of times before reaching the engine face 82 and reflectingtherefrom. This deflecting and reflecting phenomena tends to attenuate,absorb, or dissipate the energy and direct it on a line other than backto the transmitter. The phenomena of microwave absorption is facilitatedby the materials of construction of the air inlet duct 80.

The cross sectional details of the air inlet duct 80 are shown in FIG. 9in which the inner and outer faces 86, 88 are formed of fiberglassreinforced plastic of the same type as the skins 50, 52 on fuselage andwing. These skins are 0.03 to 0.06 inch thick. The fiberglas sheets areapplied to a structural foam 90 one-half or more inches in thicknessfilled with carbon as a microwave energy absorber. The foam 90 is of thesame material as the foam blanket 30. The foam 90 has a dielectricconstant of 4 to 8. The walls of the air inlet duct 80 are supported byother structure, preferably nonmetallic, not illustrated. It will beunderstood that microwave energy is absorbed as it impinges on the foamwalls of the air inlet duct 80.

The exposed upright edge or lip 94 of inlet duct 80 is tipped back fromthe vertical, FIG. 5, to direct reflection of microwave energy upwardly.The requirements of aerodynamics, engine efficiency, and so forth, willdetermine the extent this lip 94 is inclined or tilted from thevertical, but the greater the better from the consideration ofreflection. It usually will be possible to effect a compromise in whichthe lip 94 is tilted at least ten degrees to the vertical. The drawingillustrates an angle of twenty degrees from the vertical.

FIGS. 6, 7, and 8 show the means employed to avoid reflection ofmicrowave energy from areas within a cockpit canopy 91 back toward amicrowave energy source. The canopy 91 has side walls 92 and front walls93, which are inclined at angles to the vertical whereby reflectiontherefrom will be in an upward direction in normal horizontal flightdisposition of the aircraft 10.

The walls 92, 93 of the canopy 91 are made reflective by a laminatedstructure in which inner and outer acrylic laminates 95, 96 have acommon interlaminar area therebetween occupied by a thin layer of metal98. The metal 98 is then enough to be optically transparent and thickenough to be electrically effective. This is achieved by a metal coatinghaving a monomolecular thickness. The metal coating may be vapordeposited. Gold is a suitable metal for serving as an electrical filter.This filter is particularly effective on microwave energy above about2,000 mc.

There may be instances where not all of the foregoing techniques need beused, but for the accomplishment of the purposes of the reconnaissanceand surveillance vehicle 10, all of the teachings are desired to reduceto an acceptable level radar reflection back to its source. Adesideratum is to have no microwave reflection back to its source, butwhere this is not possible it is better to have only intermittentmicrowave reflection or low grade microwave reflection than to have amore readily detectable microwave reflection.

In summary, the foregoing teachings include the use of pigmented filtersfor electrical shaping, the use of filled foam for energy absorption,and the design of angular relationships relative to a line from amicrowave transmitter to avoid return of signals thereto. The total maybe said to be more than the sum of the parts in that the structureherein disclosed is the result of compromises among a number ofdesirable expedients that are at times inconsistent. The resultingstructure provides sufficient minimization of reflection to a radarsource to be acceptable, and the best configuration known in the art fora reconnaissance and surveillance vehicle.

It will be understood that this invention can be modified to adapt it tovarious circumstances and conditions, and it is accordingly desired tocomprehend within the purview of this invention such modifications asmay be considered to fall within the scope of the appended claims.

We claim:
 1. A structure for minimizing reflection of microwave energyback to its source comprising:a layer of structural plastic foam;microwave energy absorbing material filling said foam, and said filledfoam having a dielectric constant of between two and ten and having aloss tangent of between 0.05 and unity and having a resistivity ofbetween one hundred and ten thousand microohms per centimeter; and askin of thin plastic material reinforced with non-metallic fibercovering said filled structural plastic foam.
 2. A structure as setforth in claim 1, wherein said microwave energy absorbing material iscarbon.
 3. A structure as set forth in claim 1, further comprising afilter in juxtaposition with said layer of filled structural plasticfoam.
 4. A structure as set forth in claim 3, wherein said filtercomprises paint having a pigment including oxidized aluminum particles.5. A structure as set forth in claim 3, further comprising a secondlayer of filled structural plastic foam, and said layers of filledstructural plastic foam are graduated in dielectric constant andoriented in such a manner that the one of said layers having a lowerdielectric constant is positioned closer to the source of microwaveenergy than the other of said layers.